Use of martensitic precipitation hardening stainless steel

ABSTRACT

The invention relates to the use of a chrome-nickel martensitic precipitation hardening stainless steel having the following composition: 10-14 mass % chromium, 7-11 mass % nickel, 0.5-6 mass % molybdenum, 0.5-4 mass % copper, 0.05-0.55 mass % aluminium, 0.4-1.4 mass % titanium, up to 0.3 mass % carbon+nitrogen, less than 0.05 mass % sulfur, less than 0.05 mass % phosphorus, up to 0.5 mass % manganese, up to 0.5 mass % silicium, up to 0.2 mass % tantalum, niobium, vanadium and tungsten, respectively, possibly 0.9 mass % cobalt, possibly 0.0001-0.1 mass % boron, the rest being iron and ordinary impurities. Said steel is used for producing surgery implants and osteosynthesis products which are pointwisely or long-lastingly used outside and inside a body.

TECHNICAL ENVIRONMENT

The invention concerns the novel use of precipitation hardenable, martensitic, stainless steels, in particular their use in medical applications, such as implants and osteosynthesis products for application and remaining in or on the human body.

BACKGROUND AND TECHNICAL PROBLEM

Precipitation hardenable, martensitic, stainless steels are known from WO 93/07303. This describes a composition of a stainless steel, which simultaneously combines very high resistance with good ductility. This steel is described as being particularly suitable for the manufacture of injection cannulae, dental instruments and medical instruments.

In WO 01/14601 A1 a method is presented for manufacturing medical or dental instruments through a series of process steps including precipitation hardening, tempering, quenching and hardening, the result of which is a homogenous hardness of at least 450 HV.

It is, by way of example, stated that a precipitation hardenable, martensitic, stainless steel can be used for manufacturing medical instruments with the process specified there.

For a specific application of a steel in medical applications such as implants and osteosynthesis products there is a number of boundary conditions that must be met.

Depending on the purpose of their application implants and osteosynthesis products like plates, and fixing elements, such as screws and nails, have very unfavourable geometries, i.e. an unfavourable ratio of length to thickness and/or diameter for example. Because of their unfavourable geometry such implants and osteosynthesis products are very sensitive to the frequent light or heavy bending loads that are applied in practical use. Even the slightest, hardly noticeable bending of implants and/or osteosynthesis products can lead to them breaking the next time a load is applied. Because of the often very high bending loads in practice this occasionally leads to the implants and osteosynthesis products breaking. This means that not only do they have a short lifetime, but there is also a risk of injury to the patient.

There is thus an urgent need for implants and osteosynthesis products that have a high hardness, are resistant to corrosion and at the same time compared to known implants and osteosynthesis products are fracture-proof. Apart from hardness and security against fracture, simultaneous resistance to corrosion and biocompatibility are of decisive importance. Examples of corrosive media in use are blood and other body fluids. If, therefore, such implants and osteosynthesis products are damaged by corrosion or attacked, there is a danger that patients will be contaminated with the corrosion residues and develop dangerous postoperative complications.

Another typical application is in surgery. Here metal plates, nails and screws, so-called osteosynthesis products, are often used to join bones following fractures or following operative sawing of the bones, i.e. they are implanted. These implants then normally remain until complete healing of the bone and furthermore inside the body of the patient. Known osteosynthesis products available on the market are manufactured from so-called stainless steel, such as the abovementioned steel grades. Since for such osteosynthesis products, in addition to the mechanical and corrosion properties, high demands on the biocompatibility are made, there is only a limited choice of materials available for the manufacture of these products (not all the materials shown in Table 1 below meet this condition). It has been found that the fracture resistance of the known implants, in particular after long periods of indwelling, is poor. For this reason patients are urged, initially, to be careful and only apply weak loads to bones with such osteosynthesis products after the operation. In order to prevent degeneration of the underutilised musculature, it is necessary as soon as possible with the help of the hospital exercises to slow down the degeneration, although it cannot be prevented. If the patient does not take the required care with the bone operated on or due to inappropriate movements, bending loads may be applied to the osteosynthesis products or implants and finally they may break. Not only does this result in the joining together of the bone fragments intended by the application of the implant being lost, the often sharp-edged implant can cause considerable injury to the patient. In any case, in the event of the implant fracturing a further operation on the patient to rectify the damage is necessary. Because of the low resistance of the products there is an additional danger of screwed plates coming loose through micro-oscillations thus prolonging healing of the bone.

Thus there is an urgent need for stable, corrosion-resistant and biocompatible osteosynthesis products or implants which simultaneously combine high strength values with good ductility properties.

Currently a range of well known and well researched types of alloys are used for forming and manufacturing such tools and implants. These alloys include martensitic stainless steels, austenitic stainless steels, and precipitation hardenable stainless steels. Each of these known alloys has a number of good material properties, such as corrosion resistance, strength, malleability and/or ductility, but each alloy also has its disadvantages and is unable to meet certain product requirements. Complex problems and disadvantages are known from the practical use of the rotary tools, surgical implants, surgical osteosynthesis products, such as surgical plates, screws and nails, available on the market today.

The table below shows the compositions of a number of frequently used steels. TABLE 1 Compositions of various known steels in % by weight; remainder iron Alloy C Si Mn S Cr Ni Mo Cu Ti N P AISI 420 0.36 0.15 0.30 <0.020 13.5 <0.3 AISI 420 F 0.22 0.58 1.58 0.175 13.0 0.8 1.2 AISI 304 0.060 0.66 1.22 0.002 18.6 8.6 0.2 ISO 5832-1-D <0.03 <1.0 <2.0 <0.01 17.5 14 2.8 <0.5 <0.1 <0.025 ISO 5832-9 0.08 <0.75 3.6 <0.01 20.5 10.0 2.5 <0.25 0.4 <0.025 Carpenter 455 0.006 0.07 0.03 0.004 11.4 8.3 <0.1 2.2 1.2 C455 (V) 0.004 0.04 0.15 0.002 11.8 9.1 <0.1 2.0 1.6 1.4108 0.31 0.68 0.41 0.002 15.54 0.16 0.97 0.41 0.017 1.4112 0.85-0.95 <1.0 <1.0 0.030 17.0-19.0 0.9-1.3 0.040

Martensitic, stainless steels, such as the AISI 420 grades, may offer a high strength, but this is not combined with ductility. Austenitic, stainless steels, such as the AISI 300 series, can offer good corrosion resistance combined with high strength and for some applications acceptable ductility, but in order to achieve the high strength, a severe cold reduction is necessary, and this means that even the semi-finished product must have a very high strength, which in turn results in poor malleability. The group of precipitation hardenable, stainless steels, contains numerous, different grades and all with different properties. They do, however, have a number of things in common: for example, the majority of them are smelted in a one-way or more usually in a two-way process in a vacuum, in which the second stage involves smelting under a vacuum. Apart from this a large quantity of precipitation-forming elements, such as aluminium, niobium, tantalum and titanium, is necessary and often as combinations of these elements. Here the term “large” means>1.5%. A large quantity favours strength but reduces ductility and malleability. One grade can be found in U.S. Pat. No. 3,408,871. This grade offers acceptable ductility of the finished product, but in association with a strength of only about 2,000 N/mm². It can also have disadvantages during the manufacture of semi-finished products. The steel, for example, is susceptible to cracking in the annealed state.

DESCRIPTION OF THE INVENTION

One aspect of the invention concerns the ( . . . ) use of precipitation hardenable, martensitic, stainless steels, with the composition mentioned below, for the manufacture of surgical implants, surgical osteosynthesis products, surgical plates, screws and nails for application and remaining in or on the human body. One surprising effect of the invention that has been noticed is that precipitation hardenable, martensitic, stainless steel is an advantage in those applications where the combination of high fracture and bending resistance with hardness and corrosion properties is of decisive importance.

A further surprising effect of the invention that has been noticed concerns the advantageous combination of good biological compatibility of the steel described below with good corrosion properties, high ductility, good malleability and exceptionally high strength of approximately 2,500 to 3,000 N/mm². This combination allows the advantageous use of this steel in medical applications, in which the material remains in the patient's body for a relatively short or long period.

The precipitation hardenable, martensitic, stainless steel described in the invention must have the composition described below: Chromium 10 to 14 Nickel 7 to 11 Molybdenum 0.5 to 6 Copper 0.5 to 4 Aluminium 0.05 to 0.55 Titanium 0.4 to 1.4 Carbon + nitrogen up to 0.3 Sulphur less than 0.05 Phosphorus less than 0.05 Manganese up to 0.5 Silicon up to 0.5 Tantalum, niobium, vanadium up to 0.2 each and tungsten Cobalt up to 9.0 if applicable Boron 0.0001 to 0.1 if applicable

-   -   with the remainder comprising iron and the normal impurities.         Examples to Describe the Inventive Properties

The tensile strength, elongation at rupture and hardness were tested on hardened solid material pieces of the same geometry in the inventively used steel and two of the steels currently used for rotary tools.

The inventive steel tested had a composition in accordance with the abovementioned preferred embodiment of 12.0% by weight chromium, 9.1% by weight nickel, 4.0% by weight molybdenum, 2.0% by weight copper, 0.9% by weight titanium, 0.35% by weight aluminium, <0.12% by weight carbon and <0.012% by weight nitrogen. The steels used for comparison were grades 1.4112 and 1.4108, the compositions of which are given above in Table 1. The specimens were solid rods with a circular section and a diameter of 4.5 mm. All the specimens tested had been precipitation hardened. The hardening of the inventive steel took place at 475° C. for 4 hours. The hardening of grades 1.4112 and 1.4108 took place in accordance with the hardening methods prescribed for these steels. The hardening of grades 1.4112 and 1.4108 took place at 1,000° C. for 40-60 minutes in a vacuum. Then both grades were cooled in nitrogen to 50° C. Material 1.4108 was left for a further 2 hours at 160° C. The hardening of each of the materials was performed in such a way that for all tested materials a comparable material hardness was achieved. Four specimens of each material were tested. The results are summarised in the following Table 2. TABLE 2 Test results, tensile test in accordance with DIN EN 10002-1 Elongation Tensile strength at rupture Rockwell Material [MPa] [%] C hardness Inventive A 1935 9.1 52/53 Inventive A 1938 9.1 52/53 Inventive C 1941 9.1 52/53 Inventive D 1946 9.1 52/53 1.4112 1989 <2 54/55 1.4112 1981 <2 54/55 1.4112 1987 <2 54/55 1.4112 2000 <2 54/55 1.4108 1323 <2 54/55 1.4108 1263 <2 54/55 1.4108 1153 <2 54/55 1.4108 1312 <2 54/55

An investigation of the fracture points of the abovementioned materials tested clearly shows that the inventive steel has an extremely tough fracture behaviour. The fracture surfaces had the form of a so-called “funnel fracture”. In comparison the materials 1.4112 and 1.4108 demonstrated so-called fissures with a brittle fracture content of almost 100%. The good elongation at rupture behaviour of the specimens in the inventive steel is accompanied by high pliability without the material breaking. The specimens can be bent many times without breaking. In comparison the specimens in materials 1.4112 and 1.4108 broke with the first bending.

Surprisingly, it transpires that the use of the inventively used steel types for the manufacture of implants and osteosynthesis products such as surgical plates, nails and screws for application and remaining in or on the human body, has particular advantages because of the exceptional elongation at rupture behaviour of the steel grade compared with the steels used previously. With the previously used steels the hardness and corrosion resistance, as well as the biocompatibility, depending on the application, were, in particular, the top priority and with regard to the rupture resistance a compromise was accepted. With the inventive use of the present steel for the manufacture of rotary tools and osteosynthesis products the disadvantages of the fracture behaviour of the products previously available on the market could now be overcome. The inventively manufactured tools and osteosynthesis products combine hardness, high corrosion resistance, good biocompatibility and exceptional fracture resistance in the manufactured products. Even when bent the products remain secure against fracture and can, as for example in implants for plastic surgery, be bent a number of times, without losing their exceptional material properties. Furthermore, the inventively used steel grades are easy to machine and easy to mill in the hardened state, which is of benefit in the manufacture of the products. A further advantage of the application of the inventively used steel for the manufacture of implants and osteosynthesis products for application and remaining in or on the human body is the relatively low hardening temperature in the region of 425 to 525° C. allowing considerable energy cost savings during manufacture. 

1. Use of a precipitation hardenable, martensitic, stainless chrome nickel steel with the following composition (in % by weight): Chromium 10 to 14 Nickel 7 to 11 Molybdenum 0.5 to 6 Copper 0.5 to 4 Aluminium 0.05 to 0.55 Titanium 0.4 to 1.4 Carbon + nitrogen up to 0.3 Sulphur less than 0.05 Phosphorus less than 0.05 Manganese up to 0.5 Silicon up to 0.5 Tantalum, niobium, vanadium up to 0.2 each and tungsten Cobalt up to 9.0 if applicable Boron 0.0001 to 0.1 if applicable

with the remainder comprising iron and the normal impurities, for the manufacture of surgical implants and osteosynthesis products for application and remaining in or on the human body. 